Fiber-optic transmission systems prevail in optical transmission circuits used in optical communication, for example. The performance of an optical transmission circuit depends much on the structure and the performance of an optical switch that is indispensable for the optical transmission circuit. Therefore, various optical switch structures have been developed.
Presently, a mechanical optical switch in which a optical fiber or prism is mechanically moved to switch from one state to another is put into practical use. However, this mechanical optical switch has numerous problems. That is, the switching speed is low. The optical axes of descrete components are required to be aligned. Further, it is bulky and vulnerable to vibration.
Japanese Patent Laid-Open No. 39,630/1985 has disclosed an optical switch having no movable portion. Specifically, this switch has a polarizing splitter consisting of a beam splitter and a total reflection prism. Incident light is caused to enter this polarizing splitter, and then the divided light beams are made to enter respectively PLZT (lead lanthanum zirconate titanatae) forming grooved electrodes. The beams are then transmitted through a summing polarizer consisting of a beam splitter and a total reflection prism, and an output light beam exits from this summing polarizer. A signal voltage is applied between the grooved electrodes to provide switching action. Namely, this switch utilizes electro-optic effects.
It is claimed that this optical switch operates at a voltage of 80 to 100 V, and exhibits high performance including a switching speed of 1 .mu.s (1 Mhz) and an insertion loss of 1.4 dB. However, the components are arranged in a complicated manner, because light going out of an optical fiber passes through a lens, the polarizing splitter, an electric field-inducing wavelength plate made from PLZT, the summing polarizer, and a lens, and then the light reenters the optical fiber. Since the light travels through air between the components, a large reflection loss occurs and misalignment of optical axes also causes a large loss so that a large insertion loss takes place. In addition, complex grooved electrodes made from PLZT must be used. This makes it difficult to simplify the structure of the optical transmission circuit, to miniaturize the circuit, and to mass-produce it. Furthermore, this switch cannot achieve higher switching speeds which would be required in the future.
Another optical switch that operates at a low voltage, i.e., several voltage, and has a high switching speed of 1 GHz has been proposed. This switch has a waveguide made from LiNbO.sub.3. However, this switch also has several problems. Specifically, the insertion loss is as large as several dB. It is difficult to manufacture the switch.
As described above, the conventional optical circuit components are much larger in size than optical fibers through which signals are transmitted. Further, an optical fiber in a transmission circuit is required to be divided to install these components. The division of the fiber necessarily leads to an increase in the insertion loss. Accordingly, an optical switch which takes the form of an optical fiber, i.e., assumes an ideal form as an optical circuit element used for fiber-optic transmission system, has been required to be developed.